A wide variety of implants are placed at predetermined locations in the human body to repair or prevent damage. A stent is an implant that is generally tubular and is delivered to a pre-determined location in the human body where it is expanded radially in, for example, a vessel or lumen to maintain the patency of the vessel. Stents are widely used in body vessels, body canals, ducts or other body lumens. Stents may take the form of helically wound wire, or tube-like structures with numerous patterns defining the walls of the tubule.
Self-expanding stents are generally cut from a solid tube of superelastic material, such as Nitinol (NiTi) allowing the stent to be deformed and constrained in the deformed condition, then return to the natural condition when unconstrained. For example, one design comprises a solid tube of Nitinol cut to form a series of hoops that are joined together by a plurality of bridges. The bridges are shaped to allow for the frame to flex along its longitudinal and radial directions. The hoops comprise multiple struts. Each adjacent strut is connected together by an apex that allows the frame to assume an expanded diameter when unconstrained.
The delivery systems for stents are generally comprised of catheters with the stent constrained within the distal end of the catheter. It is highly desirable to keep the profile of the catheter as small as possible allowing for easy passage of the catheter through a body lumen. Self-expanding stents can be constrained at a reduced diameter for delivery to the deployment site. Once the stent is deployed, the catheter is removed, leaving the stent implanted at the desired location to maintain vessel patency.
A variety of techniques have been developed for holding a self-expanding stent in a reduced diameter delivery configuration while moving the distal end of the catheter to the deployment site. For example, a common self-expanding prosthesis delivery system employs a sheath to constrain the prosthesis or implant at the distal tip of a concentrically mounted shaft, contained within the sheath. The delivery system is advanced through the vascular system of a patient until it reaches a desired location where the shaft is held in place and the sheath is retracted, allowing the stent to expand.
The shaft is typically fabricated from a coiled metallic component designed to transmit compressive loads from the implant, while maintaining flexibility and minimizing bulk. In addition, the shaft is sufficiently rigid to resist buckling or kinking as the sheath and shaft are moved relative to each other producing frictional forces there-between. Similarly, the sheath or outer member is constructed from a flexible material that allows for navigation of the system through the vasculature of the patient. The sheath must also exhibit sufficient rigidity to handle the tensile load resulting from the frictional interactions between the sheath, prosthesis and shaft as the sheath is retracted toward the proximal end of the shaft.
Frictional and compressive forces limit the functionality of the delivery system. The magnitude of these forces increase with the strength and length of the constrained prosthesis and may be further increased by the addition of pharmaceutical, polymeric or other coatings resulting in an increase in the coefficient of friction between the sheath, prosthesis and shaft. For example, as the length of the stent increases, the shaft must bear a greater compressive load. This requires the shaft to either have a larger diameter or be more rigid to support the compressive load exerted by the prosthesis. Increasing the diameter or rigidity of the shaft results in increased frictional interactions between the shaft and the sheath. This will require a larger force to remove the sheath and deploy the prosthesis.
Several systems have been proposed that increase the deploying force in order to overcome the frictional forces between the shaft and sheath. For example, U.S. Pat. No. 6,113,608—Monroe discloses a delivery apparatus that employs a piston-based hydraulically operated retraction mechanism. The system of Monroe mounts a stent on a receiving region of a shaft that is enclosed by the distal end of a sheath. The sheath is coupled to a piston, located proximal to the sheath. An inflation lumen either supplies or withdraws pressurized fluid to the piston chamber. As the piston moves, the sheath retracts allowing the stent to deploy.
The system of Monroe fails to address several shortcomings inherent in a sheath deployment system. The coefficient of friction between the sheath, shaft and prosthesis remains unchanged. Merely increasing the deployment force for pulling back the sheath will not mitigate, and instead, will increase the buckling force exerted on the shaft. As the sheath is withdrawn the shaft may “kink” causing the prosthesis, or the vasculature to be damaged.
One approach to prevent the shaft from kinking is to employ a more rigid material. As the rigidity of the shaft increases, it loses flexibility. This makes it difficult to maneuver the delivery device within the vascular system of the patient. Another approach is to simply increase the diameter of the shaft. This will also make it difficult to navigate through the vasculature of the patient. It is also desirable to keep the profile of the delivery system as small as possible, especially when navigating through small vessels.
Yet another alternative is to vary the construction of the shaft along its length creating rigidity zones. For example sections of the support are constructed from a more rigid material while adjoining sections are constructed from a more flexible material. This approach provides for increased flexibility, however, the more rigid sections of the shaft could prove difficult to navigate through small vasculature. Moreover, constructing the shaft from more than one type of material is expensive. Currently, there is no apparatus, delivery system for the deployment of a prosthesis within the vasculature of a body that is highly flexible, has a low profile and can withstand high frictional and compressive forces encountered during deployment of the prosthesis. The present invention is designed to address these and other needs.